1. Field of the Invention
The present invention relates to a digital broadcasting receiving system, and more particularly, to a channel equalizing device and method of a terrestrial digital TV signal transmitted using a Vestigial Side Band (VSB) technology.
2. Discussion of the Related Art
In U.S.A., ATSC 8T-VSB transmission system is adopted and standardized in 1995 to provide a terrestrial digital broadcasting from the latter half of 1998 until now. In South of Korea, the same ATSC 8T-VSB transmission system as in U.S.A. is adopted and standardized to provide an experimental broadcasting on May 1995, and is converted into a test broadcasting system on Aug. 31, 2000, and is under broadcasting at each broadcasting service corporation since October, 2000.
Additionally, in a digital communication system, digital information of a transmission system is mapped using a symbol and each symbol is converted into an analog signal proportional to a magnitude or a phase to transmit the converted analog signal to a receiving stage over a transmission channel. While the signal reaching the receiving system (that is, ATSC 8T-VSB receiving system) passes through a multi-path transmission channel, it causes an inter-symbol-interference. Accordingly, it is essential to employ a channel equalizer for channel compensation, to recover an original signal from the distorted signal.
Generally, as a widely used channel equalizer, there is a Decision Feedback Equalizer (DFE) using a Least Mean Square (LMS) algorithm. In case where a signal is received over a multi-path channel, the DFE has a main path through which a great energy enters, and regards all signals, which enter through remaining paths, as inter-symbol-interference(ISI) or ghost signals entering through a reflection path. After that, the DFE corrects and extracts a phase and a magnitude of only signal entering through the main path and eliminates the signals entering through the remaining paths.
FIG. 1 is a block diagram illustrating a general decision feedback equalizer operating at a time domain.
In a brief description with reference to FIG. 1, a feed forward filter 101 eliminates the influence of signals of the path reaching earlier than those of the main path. Additionally, a feedback filter 102 eliminates the influence of signals of the path reaching later than those of the main path. In the figure, an adder 105 adds an output of the feed forward filter 101 and an output of the feedback filter 102 to output the added result to a decision unit 103. The decision unit 103 compares an output of the adder 105 with a predetermined reference signal level to decide a reference signal level most close to the output signal of the adder 105 as a decision value. At this time, the output of the decision unit 103 is fed-back to the feedback filter 102 and the controlling unit 104. That is, not the output of the adder 105, but the decision value of the decision unit 103 is inputted to the feedback filter 102.
Meanwhile, the controlling unit 104 receives the output of the equalizer, that is, the output of the adder 105 and the decision value of the decision unit 103 to update the coefficient of the feed forward filter 101 and the feedback filter 102.
In case where the decision unit 103 performs an correct decision, an output of the decision feedback equalizer is again inputted to the feedback filter 102 with noise eliminated from an equalizer output component. Therefore, noise enhancement is minimized, thereby providing an excellent performance in comparison to a general linear equalizer.
At this time, the decision feedback equalizer uses the LMS algorithm in such a way that a coefficient of a filter is updated. A step-size used in the LMS algorithm has an upper limit which is determined to stably operate the channel equalizer by a length of the filter and a degree of channel distortion. Within the upper limit, as the step-size is large, a compensation performance for a time-varying channel impulse response is improved at a dynamic channel whereas an excess Mean Square Error (MSE) is increased at a static channel to reduce an immunity for the noise generated at the channel.
Alternatively, at a duration not having a training sequence, a dynamic channel equalization performance and a static channel equalization performance are influenced depending on a blind algorithm, which is widely used to adapt the channel equalizer.